The quality of every design aspect can be measured using the cost-benefit principle

If the costs associated with interacting with a design outweigh the benefits, the design is poor. If the benefits outweigh the costs, the design is good.

For example: How long is too long for a person to wait for a web-page to load? The answer to this question is that it depends on the benefit of the interaction.

The often-cited maximum acceptable download time for pages on the Internet is ten seconds. However, the acceptability of download time is a function of the benefits provided by the downloaded page.

  • A high-benefit page can more than compensate for the cost of a download taking longer than ten seconds.
  • Conversely, a low-benefit page cannot compensate the cost of any download time.

Reducing interaction costs does improve the quality of the design, but to simply design within cost limits without consideration of the interaction benefits misses the point of design altogether—i.e., to provide benefit.